VLSI (Very Large Scale Integration) has, on the one hand, produced packaged ICs (Integrated Circuits) containing amounts of circuitry capable of unparalleled functionality. One the other hand, such complex parts typically have a great many signal having corresponding terminals (‘pins’) that require connection to an external environment; often many more than can readily be provided by just pins that could be disposed along the periphery of the part. One of the most common solutions to this problem is to use a BGA (Ball Grid Array) package.
The BGA style of IC packaging utilizes an array (whose outline is typically square, or perhaps rectangular, but any suitable shape is possible) of pads on the underside of the IC part that is to be attached to a PCA (Printed Circuit Assembly). A corresponding (mirror image) array of pads is formed on the PCA at the location that is to receive the IC to be mounted. Some BGAs are solid arrays, with a pad at each possible location, while others are only two or three peripheral layers deep, with an empty center devoid of pads. These pads are, of course, connected by traces within the PCA to the various power supplies, grounds and the myriad of input and output signals that cooperate with the BGA part. A patterned paste of powered solder is deposited on the pads of the BGA part and heated. Surface tension causes the formation of domes of solder commonly referred to as ‘balls.’ At a later time the part is placed onto the PCA, held in alignment and heat is applied during a conventional SMT (Surface Mount Technology) process. That re-heating causes a re-flow of the solder to create individual solder joints between each pad in the array on the part with its corresponding pad on the PCA. The result is a robust and inexpensive assembly process which is very widely used, and is almost universally used for larger ASIC (Application Specific IC) parts, as well as for various other larger ICs.
Alas, this venerable cost effective and efficient style of mechanical mounting and electrical interconnection between an IC and its host PCA presents a difficulty for electrical testing and troubleshooting. The trouble arises because the tasks of testing and troubleshooting generally require the probing of the signals at the pins of the IC, and those pins are frequently not accessible. It may happen that a pin of interest is accessible from the back side of the PCA, but this state of affairs is by no means assured. Owing to layout considerations, intervening ground planes, etc., some pads of the array on the PCA may have ‘buried’ or ‘blind’ vias that do not go all the way through to pads the back side. There are even instances of cooperating parts that have been designed to be placed directly in line in with one another (in each other's shadow, as it were, when viewed with ‘x-ray vision’) on opposite sides of the same substrate (e.g., a printed circuit board). When this occurs, both the ‘front side’ and ‘back side’ pads of even a Ball Grid Array having all or mostly through vias are covered by those parts, and no conventional actual electrical access to their pads is possible.
Unfortunately, the very increase in complexity and functionality possessed by those parts needing BGA style mounting is also accompanied by an increased need for testing during system turn-on and troubleshooting. Certain critical signals can often be deliberately made available as accessible test points during the layout process, but a wholesale leading of all pads of a BGA to such test points is undesirable, both from an efficient use of board space, and also from an electrical perspective: high speed signal are often conveyed over transmission lines, and the needed ‘tee’ connection is an unwanted stub that can distort fast signals and have a severe impact on performance.
The task or troubleshooting a malfunctioning assembly with several large ICs is made more complicated if the technician is deprived of the ability to make signal measurement. Often, the only resort is to component substitution. Vacuum tubes (long gone) and other socketed parts can often be readily exchanged on a trial basis, but it can be downright expensive and time consuming (not to mention the risk of unintentional damage) to experimentally swap out a large SMT BGA part just to see if it is the culprit. Furthermore, in many applications, sockets, unless absolutely necessary for some good reason, are deprecated as unreliable and expensive. It is therefore understandable that one would like to have a more definitive indication of guilt than “Well, this one is the biggest of the five (BGAs), so I think I'll change it first . . . ”. But probing the signals of a BGA part to obtain useful information, whether to verify proper performance or collect clues concerning the cause of a failure, is a problematic task.
Aside from probing the back side pads as they might be available (which even if they exist, may itself be a considerable aggravation from a mechanical access aspect), there is another prior art solution that involves mounting the BGA part to its environment through an intervening assembly that makes electrical connection to the necessary signals and makes them available, say in a cable or at an auxiliary array of pins or sockets. This is generally a customized assembly that is specific to the BGA it is intended to service, and as a result is expensive and not universal (usable with many different BGAs). Its use is generally restricted to use in development laboratories and depot level repair facilities for high volume activities (or repair of very high value assemblies) undertaken by major manufacturers. Such radical interposed mounting/probing assemblies are of little or no use to personnel wishing to service a PCA carrying an ASIC for which a custom intervening device is not available.
We would be pleased if there were an inexpensive, fairly reliable and nearly universal method and apparatus for probing, on the front side (i.e., the side carrying the BGA part) any of the signals of an arbitrary BGA-mounted IC, say for measurement by an oscilloscope or logic analyzer. It would especially desirable if such a probing apparatus and method did not depend upon the particulars of the IC (i.e., which pin is what signal), and were relatively independent of the number of the pads (i.e., to the ‘size’ or the array), but depended mainly on such standard things as pad size and center-to-center spacing of those pads. Such a technique for probing should be convenient, in that no special parts are required, save for a probe suited for a BGA of that pad style. The probe's deployment (and subsequent re-deployments) should require (with perhaps some practice) only a matter of about a minute or less, neglecting any time needed to access the PCA itself. Furthermore, such a technique ought to lend itself to probing more than one signal at a time, whether by multiple instances of a such an apparatus and its method, or by individual apparatuses that simultaneously contact a significant plurality of signals at one time. Finally, such a probing apparatus and method should be easy to use without undue error or confusion, and ought to assist its operator in correctly placing it into contact with an arbitrarily desired pin or pins within the interior of a BGA. In the extreme, this assistance might allow an operator to determine by an inspection (performed subsequent to probe placement) if such a probing apparatus is correctly placed, or if its placement is questioned, determine by passive observation where it is placed.
And upon reflection, we appreciate that much of what has been noted for BGA situations also applies to certain other situations where large ICs with arrays of parallel pins depending from an underside are mounted in zero insertion force sockets. There are times when it is desirable to probe inaccessible pins in that type of an environment. Perhaps a solution for the BGA case would also serve for the other.
Such desirable properties amount to quite a wish list for a probe. What to do?